Unlike a primary battery incapable of recharging, a secondary battery is a rechargeable battery and widely used in electronic equipments, such as cellular phones, laptop computers, camcorders, or the like, and electric vehicles, etc. Particularly, since a lithium secondary battery has an operating voltage of around 3.6 V, a capacity approximately three times higher than that of Ni—Cd batteries or Ni—MH batteries used as a power source for electric equipments, and excellent energy density per unit weight, the use of a lithium secondary battery is rapidly increasing.
In such a lithium secondary battery, a lithium-based oxide and a carbon material are respectively used as a positive active material and a negative active material. A lithium secondary battery includes a cell assembly having a plurality of unit cells in which a cathode plate and an anode plate, respectively coated with the positive active material and the negative active material, are stacked with a separator being interposed therebetween and a case for receiving the cell assembly together with an electrolyte by sealing.
Depending on the type of the external case, lithium secondary batteries are classified into can type secondary batteries, in which a cell assembly is received in a metal can, and pouch type secondary batteries, in which a cell assembly is received in a pouch case made of an aluminum laminate sheet.
The pouch type secondary battery is manufactured at low costs, high energy density, and can easily make a large-capacity battery pack by connecting the pouch type secondary batteries in series or in parallel. Therefore, pouch type secondary batteries are widely used as power sources for electric vehicles or hybrid vehicles.
The pouch type secondary battery has a structure in which a cell assembly connected with a plate-shaped electrode lead is sealed with an electrolyte in a pouch case. The electrode lead is partially exposed to the outside of the pouch case and the exposed part of the electrode lead is electrically connected to an apparatus using a secondary battery or is used to electrically connect multiple secondary batteries to each other.
FIG. 1 is an exploded perspective view showing a conventional pouch type lithium secondary battery.
Referring to FIG. 1, a conventional pouch type lithium secondary battery 10 includes an electrode assembly 30, a plurality of electrode tabs 40, 50 extending from the electrode assembly 30, electrode leads 60, 70 coupled to the electrode tabs 40, 50 by welding and a pouch case 20 receiving the electrode assembly 30.
The electrode assembly 30 is a power generation device in which a cathode and an anode are stacked in turn with a separator interposed therebetween and constructed in a stack type structure, a jelly-roll type structure or a stacking/folding type structure. Korean Patent Laid-open Publication No. 10-2009-0088761 (Title: A secondary battery containing jelly-roll typed electrode assembly) and Korean Patent Laid-Open Publication No. 10-2007-0047377 (Title: A rectangular type secondary battery comprising jelly-roll electrode assembly) disclose a secondary battery 10 including an electrode assembly 30 in a jelly-roll type structure. Also, Korean Patent Application Laid-Open Publication No. 10-2008-0036250 (Title: A hybrid-typed stack and folding electrode assembly and secondary battery containing the same) and Korean Patent No. 10-0987300 (registered on Oct. 6, 2010. Title: A stack and folding-typed electrode assembly and method for preparation of the same) disclose an electrode assembly 30 in a stack/folding type structure or a secondary battery 10 including the electrode assembly 30.
The electrode tabs 40, 50 are respectively extended from each electrode plate of the electrode assembly 30, and the electrode leads 60, 70 respectively make electrical connection with the extended electrode tabs 40, 50 by welding, and the electrode assembly, in which the electrode leads are electrically connected to the electrode tabs as described above, is combined with the pouch case 20 to be partially exposed out of the pouch case.
The pouch case 20 is made of a flexible packing material, such as an aluminum laminate sheet, has space for receiving the electrode assembly 30, and has an overall pouch shape.
Meanwhile, when the electrode tabs 40, 50 are respectively welded to the electrode leads 60, 70, an ultrasonic welding method which facilitates the welding of a soft heat-affected zone (HAZ) and a thin metal foil is commonly used. By using the ultrasonic welding, ultrasonic vibrations in the range from 10 kHz to 75 kHz are generated to make an ultrasonic vibration frictional heat between metals, and the metals are welded through the ultrasonic vibration frictional heat. That is, while the electrode tabs 40, 50 come into contact with the electrode leads 60, 70, the ultrasonic vibrations are applied thereto by using an ultrasonic welding apparatus to generate a frictional heat from the contact surfaces between the electrode tabs 40, 50 and the electrode leads 60, 70, and thus, the electrode tabs 40, 50 and the electrode leads 60, 70 are respectively welded to each other through the generated heat.
Generally, the cathode structures 40, 60 and anode structures 50, 70 are made of materials having different properties. Aluminum is mainly used for the cathode structures 40, 60 and copper or nickel-coated copper is commonly used for the anode structures 50, 70. That is, the cathode tab 40 and the cathode lead 60 are made of aluminum materials, and the anode tab 50 and the anode lead 70 are made of copper or nickel-coated copper.
However, due to the dual electrode structure, the welding strength of a welding surface formed between the cathode tab 40 and the cathode lead 60 (hereinafter referred to as a ‘cathode welding surface’) is different from the welding strength of a welding surface formed between the anode tab 50 and the anode lead 70 (hereinafter referred to as an ‘anode welding surface’). In other words, when the electrode tabs 40, 50 are respectively welded to the electrode leads 60, 70 through the same ultrasonic welding processes, since aluminum has a melting point lower than that of nickel or copper, it is easier to weld the cathode welding surface than the anode welding surface. Therefore, the welding strength of an anode welding surface becomes relatively lower than that of the cathode welding surface. Here, the welding strength indicates the maximum stress that a welding portion can endure.
When the welding strength of the anode welding surface is low, a contact condition between the anode tab 50 and the anode lead 70 in the secondary battery 10 may be easily damaged. That is, when the anode welding surface has a welding strength lower than the welding strength of the cathode welding surface, and in the case vibration or impact is applied to a secondary battery, the damage rate of the anode welding surface is higher than that of the cathode welding surface.
In addition, non-uniformity of the welding strength of the electrodes as described above leads to non-uniformity of contact resistance. In other words, as the welding strength of an anode welding surface is relatively low (i.e., bad contact condition), the contact resistance generated from the anode welding surface is higher than that generated from the cathode welding surface. The high contact resistance of a welding surface generates heat in a battery cell, and the electric conductivity of the secondary battery 10 deteriorates. Also, the non-uniform contact resistance of both electrodes causes irregular heating or side reaction which accelerates the degradation rate of the secondary battery 10.
To solve the foregoing problems, improvement is needed to enhance the welding strength for the welding surfaces between the anode tab 50 and the anode lead 70.